The use of coolants

When coolants are used

Coolants are often used on processes where metal is being cut. Usually on a machine the coolant is in the form of a liquid.

Coolants are used on some metal cutting operations when they are done by hand In these cases the coolant might be a liquid (though not soluble oil) or a solid form of what is often avaiailbable as a liquid.

Examples of this are

Tapping, threading and reaming.

Some hand operations are not usually done with coolant though the machine version might be.

For example

Sawing , filing

Coolants are used  for three main reasons:

To keep the cutter and workpiece cool.

To lubricate the cutting action.

To remove the swarf.

Types of coolants

When cutting metals it is very common to use a coolant. This leaves four options:

Do not use coolant

This is always the case for cast iron

Use coolant

For aluminium use paraffin

Soluble oil

Neat oil

There are two main types of coolant. The first is what is commonly called “soluble oil” . The other is “neat” cutting oil.

Soluble oil is exactly what is says. It is an oil that is soluble in water. The most common of these is what was Shell Dromus oil (1).  When this is supplied in a bottle it looks just like a transparent, oily liquid. It is not used like this. It is added to water in a ratio of about 1:40. At first, it forms something like curds. As it is stirred up it dissolves completely to form a white liquid, looking something like milk.

Since the Dromus oil is not particularly expensive and is used in a very dilute form it is, effectively, very cheap.

Neat cutting oils can be, just like motor oils, natural mineral oils or semi-synthetic oils. Natural mineral oils are more expensive that soluble oil and semi- synthetic ones are much more expensive.

Application of coolants

Where coolants are used they can be applied be applied simply by using a traditional oil can. This can be useful where coolants are not used all the time but are used when using them makes a significant difference for example when reaming or parting of.

In general, however cheap, all coolants are recycled. It is not just a matter of cost of the coolant that has to be considered but also the cost of disposing of it.

Where coolants are used routinely then it is done using a system where it is recirculated. The usual arrangement is that there is a tank, often within the machine, that contains the coolant. There is a pump that is used to pump the coolant. The coolant is applied via a system of pipes to the where the cutting is taking place. There is usually a tap so the flow can be controlled. The way the coolant hits the workpiece can be done by a single nozzle, or multiple nozzles.

It might seem only the place where the cutting is happening needs the coolant. But if the job produces a lot of heat it might be desirable to let the coolant flow all over the workpiece in order to dissipate the heat.

After the coolant has come off the workpiece it is collected by falling into a tray. It is filtered so that the larger pieces of swarf are removed and it is returned to the tank.

It is inevitable that oil that is used for lubricating the machine being used will, over time, contaminate the coolant. With soluble oil this will dissolve in the soluble oil whilst it is being pumped around. If it is left for any time the oil will form a layer on top of the soluble oil.

It is said that if soluble oil is used for a long time it will start to smell because of bacterial infection. It would seem that modern soluble oil is treaded to prevent this. (This is probably said by those who still (in2015) are using whale oil.)

In industry it appears to be common practice to remove this oil.

Over time, some coolant will be lost part by escaping from the recycling system and part by evaporation of the water in it. This means it has to be topped up. But through the water may escape by evaporation the salts that are in it do not and so will slowly build up over time. This means that all of the coolant must be replaced from time to time. It also pays to clean the tank out since it will slowly fill with swarf that got through the filter.

Old machines might have a system of metal pipes with “flexible” joints to distribute the coolant.

These have to be considered to be totally obsolete.

Easily the best way of distributing the coolant is by means of “Locline” type pipes and fittings.

Fig Locline piping

LocLine make a vast range of fitting, in several sizes and including every item anyone could possibly need.

The Locline piping consists of small parts that click together. There are tools for forcing them together and forcing them apart.

They can usually be forced apart by bending them too far.

They can be forced together by placing a string of them on a metal rod held vertically in a vice, putting another on top, the right way round,  and putting the jaws of a pair of pliers on the top and then banging the pliers with a hammer.

fig fitting Locline bits together

For these parts to work they need to be made very accurately and of the right material. One of the key requirements is that the pipe does not droop over time.

Needless to say there are also cheaper copies. If using these check  they fit together and the joints are rigid.

“Goodness” of coolants

Despite the vast amount of work that has been done on the theory of metal cutting there is no measure of how good a coolant actually is that is ever quoted by the seller.

Features to watch out for are

  • price
  • metals it is suitable for

is it recommended for the metal being cut?

Does it stain the metal being cut?

  • processes it is suitable for – turning and milling and/or grinding
  • transparent through to opaque

Coolants for particular metals

It is often said that cast iron can be machined perfectly well without using a coolant. However when machining cast iron it produces a large amount of very fine dust. If a coolant is used this dust is not free to enter the atmosphere.

Aluminium is best machined with paraffin.

Most every day metals that are not difficult to machine can often be machined without coolant. These would be mild steel, aluminium, brasses, some bronzes. Even so some operations parting off, drilling, tapping, threading and reaming should always be done with coolant.

1 It would appear that many  of the cutting oils formerly sold under the “Shell” name are now made by Houghton.

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